c++: Name lookup for modules

c++: Name lookup for modules

This augments the name lookup with knowledge about the BINDING_VECTOR.
That holds per-module namespace bindings, and we need to collect the
bindings in visible imports when we do lookup.  We also need to do
some checking when we're pushing a new decl to check we're not
overriding an existing visible binding in some way.

To deal with the Global Module and Module Partitions, we reserve 1 or
2 slots inthe BINDING_VECTOR to record those entities that may
legitimately appear in more than one module.

As mentioned before, the BINDING_VECTOR is created lazily, when
imported bindings appear.  The current TUs decls then appear on slot
zero.

	gcc/cp/
	* cp-tree.h (visible_instantiation_path): Renamed.
	* module.cc (get_originating_module_decl, lazy_load_binding)
	(lazy_load_members, visible_instantiation_path): Stubs.
	* name-lookup.c (STAT_TYPE_VISIBLE_P, STAT_VISIBLE): New.
	(search_imported_binding_slot, init_global_partition)
	(get_fixed_binding_slot): New.
	(name_lookup::process_module_binding): New.
	(name_lookup::search_namespace_only): Search BINDING_VECTOR.
	(name_lookup::adl_namespace_fns): Likewise.
	(name_lookip::search_adl): Search visible instantiation path.
	(maybe_lazily_declare): Maybe lazy load members.
	(implicitly_exporT_namespace): New.
	(maybe_record_mergeable_decl): New.
	(check_module_override): New.
	(do_pushdecl): Deal with BINDING_VECTOR, check override.
	(add_mergeable_namespace_entity): New.
	(get_namespace_binding): Deal with BINDING_VECTOR.
	(do_namespace_alias): Call set_originating_module.
	(lookup_elaborated_type_1): Deal with BINDING_VECTOR.
	(do_pushtag): Call set_originating_module.
	(reuse_namespace): New.
	(make_namespace_finish): Add FROM_IMPORT parm.
	(push_namespace): Deal with BINDING_VECTOR & namespace reuse.
	(maybe_save_operator_binding): Save when module CMI in play.
	* name-lookup.h (add_mergeable_namespace_entity): Declare.